Sterols are present in nearly all eucaryotic cells where these compounds serve in a variety of ways. Sterols are found as structural elements in biomembranes, are precursors for bile acids and the steroidal hormones, and act as agents for the transport of fatty acids. The sterol biosynthetic pathway remains an active area of investigation and, more recently, the regulatory aspects of this process have come under scrutiny. The possible involvement of cholesterol in vascular disease has provided great impetus for a more definitive understanding of the mechanisms involved in the control of cholesterol biosynthesis. In the past, model biological systems, such as microorganisms, have aided in such studies due to the ease of environmental control and to reduced complexity. The early biosynthetic steps in the formation of triterpenoids are similar in all organisms; therefore, it might be anticipated that the regulatory mechanisms noted in several diverse biological species may be similar. An exploration of the control mechanism for squalene: tetrahymanol cyclase found in Tetrahymena pyriformis may prove to be of value for a more general understanding of the agents and factors involved in the regulation of enzymes and, hence, metabolism. Such knowledge will allow a rational approach to pharmacological and therapeutic studies concerning sterol biosynthesis.